The bizzare part is that she's been travelling and engaging in society for the past 50 years but still doesn't quite get some stuff - not that she needs things explained in meals.

It makes sense that she wouldn't remember currency from Flamme's time, but we're lead to believe that she's been secretly training for 1000 years before Himmel went on a journey with her for 10 years. Then she went around the world collecting spells before coming back for another run.

She remembers geographic locations, and which family ran which town, as well as suits of armour that she had once allied with. It actually makes more sense for Frieren to remember currency than not. It doesn't change that quickly.

I feel the same way about the Mimic gag. It's funny and quirky, but realistically Frieren should be handling them fine. After your first Mimic, everyone knows to attack the chest before you open them.